Vertical NPNP Structure In a Triple Well CMOS Process

ABSTRACT

A vertical NPNP structure fabricated using a triple well CMOS process, as well as methods of making the vertical NPNP structure, methods of providing electrostatic discharge (ESD) protection, and design structures for a BiCMOS integrated circuit. The vertical NPNP structure may be used to provide on-chip protection to an input/output (I/O) pad from negative-voltage ESD events. A vertical PNPN structure may be also used to protect the same I/O pad from positive-voltage ESD events.

BACKGROUND

The present invention relates generally to semiconductor devicefabrication and, more particularly, to electrostatic discharge (ESD)devices for protecting an Input/Output pad in radio frequency and analogmixed signal applications, as well as methods of forming ESD protectiondevices and design structures for BiCMOS integrated circuits.

Modern electronics achieve high levels of functionality in small formfactors by integrating multiple functions onto a single chip. A commonfabrication process that allows high levels of integration at arelatively low cost is Complementary Metal-Oxide-Semiconductor (CMOS).CMOS processes build a combination of p-type and n-typemetal-oxide-semiconductor field-effect transistors (MOSFETs) toimplement logic gates and other types of digital circuits. Byintroducing additional fabrication steps into a CMOS process formingMOSFETs, bipolar junction transistors can also be formed in a processcommonly known as BiCMOS. BiCMOS processes are often used to fabricatesingle-chip mixed signal integrated circuits when the performancerequirements of the analog section cannot be met by MOSFETs alone.

Chips are often exposed to electrostatic discharge (ESD) events leadingto potentially large and damaging currents within the integratedcircuit. Increasing integration densities and performance demands onCMOS and BiCMOS chips have resulted in reduced device dimensions, whichhave increased the susceptibility of the integrated circuit to ESD.Manufacturers, assemblers, and users of integrated circuits must takeprecautions to avoid ESD. For example, ESD prevention can be part of theintegrated circuit itself and may include special design techniques forinput and output pins.

The problem of providing effective ESD protection in mixed signalapplications is further complicated by the necessity of handling analogsignals. Analog signals typically present a wider range of voltages thandigital signals, thus requiring mixed signal chips to tolerate largerinput voltages before ESD protection devices are triggered. This reducesthe voltage window for ESD device operation, placing a furtherconstraint on the types of devices that can be used. High frequencyapplications further require that ESD protection devices provide lowcapacitive loading of the input.

Therefore, there is a need for improved ESD protection devices that canprotect an integrated circuit from negative and/or positiveelectrostatic discharges, as well as methods of making ESD protectiondevices and design structures for BiCMOS integrated circuits.

BRIEF SUMMARY

In one embodiment, a device structure includes first and second regionsof n-type semiconductor material, first and second regions of p-typesemiconductor material, and an input/output (I/O) pad electricallyconnected to the first region of n-type semiconductor material and thefirst region of p-type semiconductor material. The first region ofp-type semiconductor material is stacked vertically between the firstand second regions of n-type semiconductor material to form a verticalNPN bipolar transistor. The first and second regions of p-typesemiconductor material are stacked vertically relative to the secondregion of n-type semiconductor material to form a vertical PNP bipolartransistor. The first region of p-type semiconductor material is sharedbetween the first vertical NPN bipolar transistor and the first verticalPNP bipolar transistor. The second region of n-type semiconductormaterial is shared between the first vertical NPN bipolar transistor andthe first vertical PNP bipolar transistor. The second region of n-typesemiconductor material and the second region of p-type semiconductormaterial are configured to be electrically connected to a positive powersupply voltage such that an electrostatic discharge (ESD) pulse ofnegative voltage at the I/O pad is directed through the first verticalNPN bipolar transistor and the first vertical PNP bipolar transistor toa power supply supplying the positive power supply voltage

In another embodiment, the device structure is an ESD protection devicestructure that is included in a design structure, which is embodied in amachine readable storage medium for designing, manufacturing, or testingan integrated circuit. The design structure may comprise a netlist. Thedesign structure may also reside on storage medium as a data format usedfor the exchange of layout data of integrated circuits. The designstructure may reside in a programmable gate array.

In another embodiment, a method is provided for fabricating a devicestructure includes forming a vertical stack in a p-type substratecontaining first and second n-type semiconductor regions and a firstp-type semiconductor region that are arranged in depth to define a firstvertical NPN bipolar transistor and a first vertical PNP bipolartransistor. The p-type semiconductor region is shared between the firstvertical NPN bipolar transistor and the first vertical PNP bipolartransistor. The second n-type semiconductor region is shared between thefirst vertical NPN bipolar transistor and the first vertical PNP bipolartransistor. A first wiring path, which is formed in an interconnectstructure, is connected to the first n-type semiconductor region and thep-type semiconductor region. The method further includes forming an I/Opad connected by the first wiring path to the first n-type semiconductorregion and the p-type semiconductor region of the first vertical NPNbipolar transistor. A second wiring path, which is formed in theinterconnect structure, is configured to connect the second n-typesemiconductor region and the p-type semiconductor region of the firstvertical PNP bipolar transistor to a positive power supply voltage.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIG. 1 is a diagrammatic cross-sectional view of a vertical NPNP devicestructure in accordance with an embodiment of the invention.

FIG. 2 is a diagrammatic cross-sectional view of a vertical PNPN devicestructure built on a different another portion of the substrate used tobuild the vertical NPNP device structure of FIG. 1.

FIG. 3 is a diagrammatic schematic illustrating the electricalconfiguration of the vertical NPNP device structure in FIG. 1.

FIG. 4 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

A triple well process is typically implemented in CMOS and BiCMOSprocesses to improve isolation between devices. In a typical single wellprocess implemented on a p-type substrate, islands of n-type silicon(n-wells) may be established within a p-type substrate as a step informing CMOS devices. Additional wells may then be formed within then-wells by introducing p-type dopants in a concentration effective toform p-wells when activated. A triple well process may establishadditional n-doped wells within a p-well to form a series of alternatelydoped semiconductor islands, which allows later formed islands to beenclosed by earlier formed islands. Devices may thereby be isolated fromeach other by the depletion regions that form near the boundariesbetween n-type and p-type semiconductors. In particular, the depletionlayer between the p-type substrate and the first n-well may provideisolation between the substrate and devices formed in the n-well.Similar structures may also be implemented by starting with an n-typesubstrate and substituting a p-well for the first n-well. In anembodiment of the invention, the previously described p-n junctionformed between the n-well and the p-type substrate may be employed toform part of a vertical NPNP structure forming an SCR useful inprotecting against ESD. The vertical NPNP structure is able to toleratenormal input voltages over a wide range without reducing itseffectiveness against ESD.

With reference to FIG. 1 and in accordance with an embodiment of theinvention, a semiconductor device 10 used for negative-voltage ESDprotection includes a buried or deep n-well 14, contact regions 16, 18,and n-wells 20, 22 formed in an active region of a bulk substrate 12.The contact regions 16, 18, which intersect a top surface 13 of thesubstrate 12, are electrically coupled to the deep n-well 14 by then-wells 20, 22. The deep n-well 14, contact regions 16, 18, and n-wells20, 22 have a conductivity type opposite to the conductivity type of thesubstrate 12 and typically contain different dopant concentrations suchthat the regions have comparatively different electrical conductivities.The substrate 12 may be comprised of lightly-doped p-type semiconductormaterial, such as p⁻ silicon, and is preferably monocrystalline orsingle-crystal material.

The deep n-well 14 may be a region of substrate 12 doped with an n-typedopant at a concentration sufficient to form lightly-doped n-typesemiconductor material, such as n silicon. The n-type contact regions16, 18 may be regions of substrate 12 doped with an n-type dopant atconcentration sufficient to form heavily-doped semiconductor material,such as n⁺ silicon, with a reduced electrical resistance in comparisonwith the respective n-wells 20, 22. The n-wells 20, 22 may be regions ofsubstrate 12 doped with an n-type dopant at a concentration sufficientto form lightly-doped n-type semiconductor material, such as n⁻ silicon.The n-wells 20, 22 and contact regions 16, 18 define a conductivereachthrough region extending from the top surface 13 of the substrate12 to the deep n-well 14.

Semiconductor device 10 further includes a p-well 24 and an isolatedn-well 28 within the p-well 24. The p-well 24 is comprised of p-typesemiconductor material characterized by a conductivity type opposite tothe conductivity type of the deep n-well 14, n-wells 20, 22, and contactregions 16, 18. The n-wells 20, 22 define a ring surrounding the p-well24 for lateral isolation and the deep n-well 14 underlies the p-well 24such that the p-well 24 is isolated from the substrate 12. The p-well 24may be a region of substrate 12 lightly doped with a p-type dopant at aconcentration sufficient to form lightly-doped p-type semiconductormaterial, such as p⁻ silicon. The p-well 24 is electrically coupled to ap-type contact region 26, which may be a region of substrate 12intersecting the top surface 13 and doped with a p-type dopant at aconcentration sufficient to form heavily-doped p-type semiconductormaterial, such as p⁺ silicon, with a reduced electrical resistance incomparison with the p-well 24.

The isolated n-well 28 is enclosed within the p-well 24, which isolatesthe n-well 28 from the neighboring n-wells 20, 22 and deep n-well 14,and is therefore formed at a shallower depth than the deep n-well 14.The isolated n-well 28 is a region of substrate 12 doped with an n-typedopant at a concentration sufficient to form lightly-doped n-typesemiconductor material, such as n⁻ silicon. The isolated n-well 28 iselectrically coupled with an n-type contact region 30, which is a regionof substrate 12 intersecting the top surface 13 and doped with an n-typedopant at a concentration sufficient to form heavily-doped n-typesemiconductor material, such as n⁻ silicon, characterized by a reducedelectrical resistance in comparison with the isolated n-well 28.Additional contact regions 32, 34 formed of heavily-doped semiconductormaterial, such as p⁺-doped silicon, may be electrically coupled to thesubstrate 12.

Dielectric regions 36 are defined in the substrate 10 and are configuredto laterally isolate the various contact regions 16, 18, 26, 30, 32, 34.The dielectric regions 36 may be formed by a shallow trench isolation(STI) technique that relies on a lithography and dry etching process todefine closed-bottomed trenches in substrate 12, fills the trenches withdielectric material, and planarizes the layer to the top surface 13 ofthe substrate 12 using a chemical mechanical polishing (CMP) process.The dielectric material may be an oxide, such as densifiedtetraethylorthosilicate (TEOS) deposited by thermal chemical vapordeposition (THCVD) or a high-density plasma (HDP) oxide deposited withplasma assistance.

The isolated n-well 28, p-well 24, deep n-well 14, and substrate 12define a vertical NPNP structure consisting of a vertical NPN bipolartransistor Q1 and a vertical PNP bipolar transistor Q2. The verticalNPNP structure has a vertical arrangement along an axis normal to thetop surface 13 of the substrate 12. A first p-n junction is formed alongan interface between the isolated n-well 28 and p-well 24. A second p-njunction is formed along an interface between the p-well 24 and deepn-well 14. A third p-n junction is formed along an interface between thedeep n-well 14 and substrate 12.

The vertical NPN bipolar transistor Q1 may be formed from the top threeregions of the NPNP structure, with transistor Q1 having an emitterregion or cathode defined by isolated n-well 28, a gate or base regiondefined by p-well 24, and a collector region or anode defined by deepn-well 14. The vertical PNP bipolar transistor Q2 may be formed havingan emitter region or cathode defined by the semiconductor substrate 12,a gate or base region defined by deep n-well 14, and a collector oranode defined by p-well 24. Transistors Q1 and Q2 of semiconductordevice 10 may thereby collectively share the p-well 24 and deep n-well14 and form a vertical Silicon Controlled Rectifier (SCR) device.

An input/output (I/O) pad 38 may be electrically coupled to contactregions 26, 30 so that the base and cathode of the vertical NPN bipolartransistor Q1 are connected in series and shorted. Contact regions 18,34 may be electrically coupled to a power pad 40 so that the base andanode of the vertical PNP bipolar transistor Q2 are connected in seriesand shorted. The power pad 40 is electrically connected to a terminal ofa power supply 47 that supplies a substantially constant voltage sourcesuch as, for example, a supply voltage V_(DD), or a reference voltagesource that is different than V_(DD) such as, for example, a voltagethat is greater than V_(DD).

In this manner, an ESD path for a negative-voltage ESD pulse 41 isdefined through the transistors Q1, Q2 from the I/O pad 38 to the pad 40connected with the supply voltage V_(DD) of the power supply 47. Due toits vertical configuration, transistor Q2 may have a shorter base lengththan a horizontally configured PNP transistor as known in the art. Thebreakdown voltage may be selected by adjusting the doping levels ofp-well 24 and deep n-well 14.

The SCR defined by the semiconductor device 10 provides ESD protectionfor negative-voltage ESD pulses at the I/O pad 38. The SCR remains in ahigh impedance state until a negative voltage from the ESD pulse 41across it exceeds a threshold voltage, at which point the SCR switchesto a low impedance state. Once triggered, the SCR remains in the lowimpedance state until the current of the negative voltage ESD pulse 41flowing through it drops below a threshold—known as the holdingcurrent—thereby remaining on until the ESD pulse 41 has dissipated.

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with another embodiment of theinvention, a semiconductor device 50 used for positive-voltage ESDprotection includes, a buried deep n-well 54, n-type contact regions 56,58, and n-wells 60, 62 formed in a different active region of thesubstrate 12 than device 10 (FIG. 1). The contact regions 56, 58 areelectrically coupled to the deep n-well 54 by the n-wells 60, 62. Thecontact regions 56, 58 and n-wells 60, 62 define a conductivereachthrough region extending from the top surface 13 of the substrate12 to the deep n-well 54. The deep n-well 54, contact regions 56, 18,and n-wells 60, 62 have a conductivity type opposite to the conductivitytype of the substrate 12 and typically contain different dopantconcentrations such that the regions have different electricalconductivities.

The deep n-well 54 is a region of substrate 12 doped with an n-typedopant at a concentration sufficient to form lightly-doped n-typesemiconductor material, such as n⁻ silicon. The contact regions 56, 58are comprised of regions of substrate 12 that intersect the top surface13 and are doped with an n-type dopant at a concentration sufficient toform heavily-doped n-type semiconductor material, such as n⁺ silicon.The n-wells 60, 62 are regions of substrate 12 doped with an n-typedopant at a concentration sufficient to form lightly-doped n-typesemiconductor material, such as n⁻ silicon. The n-wells 60, 62physically comprise a continuous ring-shaped region of lightly-dopedsemiconductor material and intersect the top surface 13 of the substrate12.

Semiconductor device 50 further includes a p-well 64 and an isolatedn-well 68 within the p-well 64. The p-well 64 is comprised of p-typesemiconductor material characterized by a conductivity type opposite tothe conductivity type of the deep n-well 54, n-wells 60, 62, and contactregions 56, 58. The N-wells 60, 62 define a ring surrounding the p-well64 for lateral isolation and the deep n-well 54 underlies the p-well 64such that the p-well 64 is isolated from the substrate 12. The contactregions 56, 58 likewise physically comprise a continuous ring-shapedregion of heavily-doped semiconductor material. The p-well 64 is aregion of substrate 12 doped with a p-type dopant at a concentrationsufficient to form lightly-doped p-type semiconductor material, such asp⁻ silicon. The p-well 64 is electrically coupled to a p-type contactregion 66, which is a region of substrate 12 intersecting the topsurface 13 of the substrate 12 and doped with a p-type dopant at aconcentration sufficient to form heavily-doped n-type semiconductormaterial, such as p⁺ silicon.

The isolated n-well 68 is enclosed within the p-well 64 so that then-well 68 is electrically isolated from the other n-wells 60, 62 anddeep n-well 54. The isolated n-well 68 is a region of substrate 12 dopedwith an n-type dopant at a concentration sufficient to formlightly-doped n-type semiconductor material, such as n⁻ silicon. Theisolated n-well 68 is electrically coupled with an n-type contact region70. Contact region 70 is region of substrate 12 electrically coupledwith isolated n-well 68 and doped with an n-type dopant at aconcentration sufficient to form heavily-doped n-type semiconductormaterial, such as n⁺ silicon. A p-type contact region 72 is also formedin the isolated n-well 68 region of substrate 12 and is doped with ap-type dopant at a concentration sufficient to form heavily-doped p-typesemiconductor material, such as p⁺ silicon. Additional contact regions74, 76 formed of heavily-doped semiconductor material, such as p⁺-dopedsilicon, may be electrically coupled to the substrate 12. The contactregions 70, 72, 74, 76 each intersect the top surface 13 of thesubstrate 12. Dielectric regions 78, which are structurally similar todielectric regions 26 and formed in a similar manner, laterally isolatecontact regions 56, 58, 66, 70, 72, 74, 76.

The contact 72, isolated n-well 68, p-well 64, and deep n-well 54 definea vertical PNPN structure consisting of a vertical PNP bipolartransistor Q3 and a vertical PNP bipolar transistor Q4. The verticalPNPN structure has a vertical arrangement along an axis normal to thetop surface 13 of the substrate 12. A first p-n junction is formed alongan interface between the contact 72 and isolated n-well 68. A second p-njunction is formed along an interface between the isolated n-well 68 andp-well 64. A third p-n junction is formed along an interface between thep-well 64 and deep n-well 54.

The vertical PNP bipolar transistor Q3 may be formed from the top threeregions of the PNPN structure, with transistor Q3 having an emitterregion or cathode defined by contact region 72, a base region defined byisolated n-well 68, and a collector region or anode defined by p-well64. The vertical NPN bipolar transistor Q4 may be formed having anemitter region or cathode defined by the deep n-well 54, a base regiondefined by p-well 64, and a collector or anode defined by isolatedn-well 68. Transistors Q3 and Q4 of semiconductor device 50 may therebycollectively share the isolated n-well 68 and p-well 64 and form avertical SCR device.

The I/O pad 38 is electrically coupled to contact regions 70, 72 so thatthe base and anode of the vertical PNP bipolar transistor Q3 areconnected in series and shorted. Contact regions 58, 66 are electricallycoupled to the power pad 40 so that the base and cathode of the verticalNPN bipolar transistor Q4 are connected in series and shorted. In thismanner, an ESD path for a positive-voltage ESD 71 is defined through thetransistors Q3, Q4 from the I/O pad 38 to the power pad 40 coupled withthe terminal on a power supply 49 that supplies the negative supplyvoltage V_(SS).

The PNPN SCR defined by the semiconductor device 50 provides ESDprotection for positive voltage ESD pulses at the I/O pad 38. The SCRremains in a high impedance state until a positive voltage from the ESDpulse 71 across it exceeds a threshold voltage, at which point the SCRswitches to a low impedance state. Once triggered, the SCR remains inthe low impedance state until the current of the positive-voltage ESDpulse 71 flowing through it drops below the holding current, therebyremaining on until the ESD pulse 71 has dissipated.

The device structures of FIGS. 1 and 2 may be formed utilizing standardCMOS or BiCMOS fabrication process steps applied during the fabricationof transistor structures on the substrate 12. As understood by a personhaving ordinary skill in the art of semiconductor fabrication, thesestandard CMOS or BiCMOS fabrication process steps may dope differentregions of substrate 12 by masking, implanting dopants of suitableimpurity species with suitable kinetic energies, and thermallyactivating the implanted dopants. Formation of the regions of thedevices 10, 50 may be performed simultaneously with formation of thewells and contacts used in the fabrication of CMOS PFETs and/or NFETsused in the functional circuits of an integrated circuit on substrate 12to be protected by the SCR ESD protection devices whose fabrication isbeing described. In particular, formation of the wells of the devices10, 50 may proceed concurrently with the formation of n-wells andp-wells of triple-well structures in other regions of the substrate 12that are used to fabricate CMOS PFETs and/or NFETs.

In one fabrication sequence, the dielectric regions 36 for device 10 andthe dielectric regions 78 for device 50 are formed by standard CMOS orBiCMOS process steps in the substrate. The dielectric regions 36, 78 maybe formed by a conventional STI process and may be formed concurrentlywith similar dielectric regions (not shown) for other device structureson the substrate 12.

The deep n-well 14 of device 10 and the deep n-well 54 of device 50 areconcurrently created in the p-type silicon substrate 12 by applying apatterned mask layer on the substrate 12 and implanting ions ofPhosphorous (P), Arsenic (As), Antimony (Sb), or other suitable n-typedopant at one or more selected kinetic energies into the semiconductormaterial of the substrate 12. Windows in the patterned mask layer exposethe substrate 12 at the intended locations for the deep n-wells 14, 54.During each ion implantation, the patterned mask layer blocks the ionsof n-type dopant from penetrating into the substrate 12 outside of thewindows. Each implantation includes an ion dose selected to yield adesired dopant concentration. Similar windows may be provided in thepatterned mask layer for implanting deep n-wells of triple wellstructures in other regions of the substrate 12 that are used tofabricate CMOS PFETs and/or NFETs.

The n-wells 20, 22 of device 10 and the n-wells 60, 62 of device 50 maybe concurrently created in the p-type silicon substrate 12 by applying apatterned mask layer on the substrate 12 and implanting ions of ann-type dopant at one or more selected kinetic energies into thesemiconductor material of the substrate 12. Windows in the patternedmask layer expose the substrate 12 at the intended locations for then-wells 20, 22, 60, 62. During each ion implantation, the patterned masklayer blocks the ions of n-type dopant from penetrating into thesubstrate 12 outside of the windows. Each implantation includes an iondose selected to yield a desired dopant concentration. Similar windowsmay be provided in the patterned mask layer for implanting reach-throughn-wells of triple well structures in other regions of the substrate 12that are used to fabricate CMOS PFETs and/or NFETs.

The p-well 24 of device 10 and the p-well 64 of device 50 may beconcurrently created in the p-type silicon substrate 12 by applying apatterned mask layer on the substrate 12 and implanting ions of Boron(B), Aluminum (Al), Gallium (Ga), or any other suitable p-type dopant atone or more selected kinetic energies into the semiconductor material ofthe substrate 12. Windows in the patterned mask layer expose thesubstrate 12 at the intended locations for the p-wells 24, 64. Duringeach ion implantation, the patterned mask layer blocks ions frompenetrating into the substrate 12 outside of the windows. Eachimplantation includes an ion dose selected to yield a desired dopantconcentration. Similar windows may be provided in the patterned masklayer for implanting p-wells of triple well structures in other regionsof the substrate 12 that are used to fabricate CMOS PFETs and/or NFETs.

The isolated n-well 28 of device 10 and the isolated n-well 68 of device50 may be concurrently created in the p-type silicon substrate 12 byapplying a patterned mask layer on the substrate 12 and implanting ionsof an n-type dopant at one or more selected kinetic energies into thesemiconductor material of the substrate 12. Windows in the patternedmask layer expose the substrate 12 at the intended locations for theisolated n-well 28, 68. During each ion implantation, the patterned masklayer blocks the ions of n-type dopant from penetrating into thesubstrate 12 outside of the windows. Similar windows may be provided inthe patterned mask layer for implanting isolated n-wells of triple wellstructures in other regions of the substrate 12 that are used tofabricate CMOS PFETs and/or NFETs.

The n-type contact regions 16, 18, 30 of device 10 and the n-typecontact regions 56, 58, 70 of device 50 may be concurrently created inthe p-type silicon substrate 12 by applying a patterned mask layer onthe substrate 12 and implanting ions of an n-type dopant at one or moreselected kinetic energies into the semiconductor material of thesubstrate 12. Windows in the patterned mask layer expose the substrate12 at the intended locations for the n-type contact regions 16, 18, 30,56, 58, 70. During each ion implantation, the patterned mask layerblocks the ions of n-type dopant from penetrating into the substrate 12outside of the windows. Similar windows may be provided in the patternedmask layer for implanting n-type contact regions of triple wellstructures in other regions of the substrate 12 that are used tofabricate CMOS PFETs and/or NFETs.

The p-type contact region 26 of device 10 and the p-type contact regions66, 72 of device 50 may be concurrently created in the p-type siliconsubstrate 12 by applying a patterned mask layer on the substrate 12 andimplanting ions of a p-type dopant at one or more selected kineticenergies into the semiconductor material of the substrate 12. Windows inthe patterned mask layer expose the substrate 12 at the intendedlocations for the n-type contact regions 26, 66, 72. During each ionimplantation, the patterned mask layer blocks the ions of n-type dopantfrom penetrating into the substrate 12 outside of the windows. Similarwindows may be provided in the patterned mask layer for implantingp-type contact regions of triple well structures in other regions of thesubstrate 12 that are used to fabricate CMOS PFETs and/or NFETs.

In each instance, the ion implantation mask may be a photoresist layercomprised of a sacrificial organic material and applied to the surface13 of substrate 12 by spin coating. The photolithography process entailsexposing the photoresist layer to radiation imaged through a photomaskand developing the resultant latent feature pattern in the exposedresist to define windows that expose portions of the substrate 12 at theintended locations for the implanted wells or contacts. After eachimplantation, the mask is stripped from the surface 13.

Standard back-end-of-line (BEOL) processing follows the formation ofdevices 10, 50 to form a BEOL interconnect structure. Each level of theBEOL interconnect structure may be fabricated by damascene processes,such as a dual damascene process in which a dielectric layer isdeposited, vias and trenches are etched in the dielectric layer, and thevias and trenches are filled with a conductor using a single blanketdeposition followed by planarization. The damascene process isreplicated to stack multiple wiring levels so that a multi-level, highdensity framework of conductive interconnections is formed. Damasceneprocesses and materials used in damascene processes are understood by aperson having ordinary skill in the art.

The BEOL interconnect structure includes one or more lower dielectriclayers 92 containing metallization 80, 82, 86, 88 that supplies localinterconnect wiring, and one or more upper dielectric layers 94containing metallization 81, 83, 87, 89 for levels of interconnectwiring. The metallization 80, 81 defines a wiring path coupling the I/Opad 38 with the local interconnect wiring of contacts 26, 30. Themetallization 82, 83 defines a wiring path coupling the I/O pad 38 withthe local interconnect wiring of contacts 70, 72. The metallization 86,87 defines a wiring path coupling the power pad 40 with the localinterconnect wiring for contact regions 18, 34. The metallization 88, 89defines a wiring path coupling the power pad 40 with the localinterconnect wiring for contact regions 58, 66.

The I/O pad 38 and power pad 40 are fabricated using BEOL processes,such as layer deposition, photolithography and subtractive etchingunderstood by a person having ordinary skill in the art. For example,the I/O pad 38 and power pad 40 may be fabricated by depositing a layerstack including a top layer of aluminum (Al), copper (Cu), or an alloy(e.g., AlCu) onto the topmost level of the interconnect wiring andshaping the layer stack with one or more photolithography and etchingprocesses.

With reference to FIG. 3, which presents a schematic diagram of the SCR90 formed by transistors Q1 and Q2 as shown in FIG. 1, the SCR 90 isshown connected in a breakdown configuration. In operation, the I/O pad38 is electrically coupled to contact regions 26, 30, and contactregions 18, 34 are electrically coupled to the power pad 40 and thesupply voltage V_(DD), thereby forming an ESD path for anegative-voltage ESD pulse 41 from the I/O pad 38 to the power pad 40through the SCR 90. During normal circuit operation conditions, the I/Opad 38 is biased at a voltage lower than V_(DD), which results in thep-n junction of the SCR 90 between the n-well 22 and the p-well 24 beingreversed biased and maintains the SCR 90 in a high-impedance state,sometimes referred to as a reverse blocking mode.

During an ESD event, the NPNP structure works to discharge negativecurrent from the I/O pad 38 to the positive power supply voltage V_(DD).When a negative-voltage ESD pulse 41 occurs, the voltage at I/O pad 38may drop sufficiently below the substrate bias voltage at the power pad40 to cause the reverse breakdown voltage of the p-n junction to beexceeded. The p-n junction may then break down, injecting a large numberof carriers into the p-well 24 defining the base region of transistor Q1and into the deep n-well 14 defining the base region of transistor Q2.The presence of these carriers may trigger transistors Q1 and Q2 toswitch on, providing a low impedance path from the I/O pad 38 though thetransistors Q1, Q2 to the power pad 40, which may shunt the high currentin the ESD pulse 41 safely away from the I/O pad 38 and the integratedcircuit on the substrate 12 to the supply voltage V_(DD) of the powersupply 47. Once the current path from the anode to the cathode of theSCR 90 is established, the transistors Q1, Q2—and thus the SCR 90—remainin a low-impedance state until the current from the ESD pulse 41 dropsbelow the holding current. When the ESD pulse 41 has dissipated, thecurrent through the SCR 90 drops below the holding current, and the SCR90 reverts back to a high impedance state awaiting the occurrence ofanother ESD pulse 41.

The NPNP SCR structure supplied by device 10 is capable of providing ESDprotection in RF and analog/mixed signal applications which havenon-standard voltage requirements at the I/O pad 38. In these demandingapplications, the voltage swing requirement at the I/O pad 38 may be ±10volts during normal operation. The relatively large voltage on the I/Opad 38 during normal operation prevents the effective use ofconventional ESD devices, such as grounded gate NFETs and diodes. Inparticular, the NPNP SCR structure supplied by device 10 may be used inapplications that require a large negative voltage (e.g., a voltage ofless than or equal to −5 volts) on the I/O pad 38 during normaloperation.

Referring now to FIG. 2, positive voltage ESD protection may be providedby the vertical SCR formed by transistors Q3 and Q4 by electricallycoupling I/O pad 38 to contact regions 70, 72, which may include the SCRdevice anode. Contact regions 58, 66, which may include the SCR devicecathode, may be electrically coupled to the power pad 40, therebyforming an ESD path for a positive-voltage ESD pulse 71 from the I/O pad38 to the power pad 40 and the negative power supply voltage V_(SS) ofthe power supply 47 through the PNPN SCR device 50.

During normal circuit operation condition, because the voltage on theI/O pad 38 is always higher than V_(SS), the p-n junction of the p-well64 and n-well 68 is reverse biased, which may keep the SCR device in ahigh-impedance state. During an ESD event, the PNPN structure works todischarge positive current from the I/O pad 38 to the negative powersupply voltage V_(SS). When an ESD pulse 71 occurs that drives the I/Opad 38 voltage above the breakdown voltage of the reverse biased p-njunction, the p-n junction may break down, which in turn may inject alarge number of carriers into the respective base regions of Q3 and Q4.The presence of these carriers may cause transistors Q3 and Q4 to switchinto a low impedance state, thereby shunting the ESD current to thepower pad 40. Once a current from the anode to the cathode of the SCRdevice is established, the transistors Q3, Q4—and thus the SCRstructure—may remain in a low-impedance state until the current dropsbelow the holding current. When the ESD pulse 71 has dissipated and thecurrent through the SCR device drops below the holding current, the SCRdevice may revert back to a high impedance state and await theoccurrence of another positive-voltage ESD pulse 71.

Thus, in an embodiment of the invention, both positive and negative ESDprotection may be provided by electrically coupling I/O pad 38 with thecontacts 26, 30, 70, 72 for both the NPNP and PNPN SCR structures aspreviously and separately described. However, in alternativeembodiments, the NPNP and PNPN SCR structures may be electricallycoupled with different I/O pads so that the NPNP structure protects theI/O pad 38 against negative voltage ESD pulses or events.

FIG. 4 shows a block diagram of an exemplary design flow 100 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 100 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIG. 1.The design structures processed and/or generated by design flow 100 maybe encoded on machine-readable transmission or storage media to includedata and/or instructions that when executed or otherwise processed on adata processing system generate a logically, structurally, mechanically,or otherwise functionally equivalent representation of hardwarecomponents, circuits, devices, or systems. Machines include, but are notlimited to, any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g. e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g. a machine for programming a programmable gate array).

Design flow 100 may vary depending on the type of representation beingdesigned. For example, a design flow 100 for building an applicationspecific IC (ASIC) may differ from a design flow 100 for designing astandard component or from a design flow 100 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 4 illustrates multiple such design structures including an inputdesign structure 102 that is preferably processed by a design process104. Design structure 102 may be a logical simulation design structuregenerated and processed by design process 104 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 102 may also or alternatively comprise data and/or programinstructions that when processed by design process 104, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 102 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 102 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 104 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIG. 1. As such,design structure 102 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 104 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIG. 1 to generate a Netlist 106which may contain design structures such as design structure 102.Netlist 106 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 106 may be synthesized using an iterative process inwhich netlist 106 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 106 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 104 may include hardware and software modules forprocessing a variety of input data structure types including Netlist106. Such data structure types may reside, for example, within libraryelements 108 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 110, characterization data 112, verification data 114,design rules 116, and test data files 118 which may include input testpatterns, output test results, and other testing information. Designprocess 104 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 104 withoutdeviating from the scope and spirit of the invention. Design process 104may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 104 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 102 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 120.Design structure 120 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in an IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 102, design structure 120 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIG. 1. In one embodiment, design structure 120 maycomprise a compiled, executable HDL simulation model that functionallysimulates the devices shown in FIG. 1.

Design structure 120 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 120 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIG. 1. Design structure 120may then proceed to a stage 122 where, for example, design structure120: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It will be understood that when an element is described as being“connected” or “coupled” to or with another element, it can be directlyconnected or coupled to the other element or, instead, one or moreintervening elements may be present. In contrast, when an element isdescribed as being “directly connected” or “directly coupled” to anotherelement, there are no intervening elements present. When an element isdescribed as being “indirectly connected” or “indirectly coupled” toanother element, there is at least one intervening element present.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A device structure comprising: a first region of n-type semiconductormaterial; a second region of n-type semiconductor material; a firstregion of p-type semiconductor material, the first region of p-typesemiconductor material stacked vertically between the first and secondregions of n-type semiconductor material to form a vertical NPN bipolartransistor; a second region of p-type semiconductor material, the firstand second regions of p-type semiconductor material stacked verticallyrelative to the second region of n-type semiconductor material to form avertical PNP bipolar transistor; and an input/output (I/O) padelectrically connected to the first region of n-type semiconductormaterial and the first region of p-type semiconductor material, whereinthe first region of p-type semiconductor material is shared between thefirst vertical NPN bipolar transistor and the first vertical PNP bipolartransistor, the second region of n-type semiconductor material is sharedbetween the first vertical NPN bipolar transistor and the first verticalPNP bipolar transistor, and the second region of n-type semiconductormaterial and the second region of p-type semiconductor material areconfigured to be electrically connected to a positive power supplyvoltage such that an electrostatic discharge (ESD) pulse of negativevoltage at the I/O pad is directed through the first vertical NPNbipolar transistor and the first vertical PNP bipolar transistor to apower supply supplying the positive power supply voltage.
 2. The devicestructure of claim 1 further comprising: a vertical PNPN structureelectrically coupled with the I/O pad, the vertical PNPN configured todirect a positive ESD pulse from the I/O pad to the power supplysupplying the negative power supply voltage.
 3. The device structure ofclaim 1 wherein the second region of p-type semiconductor material is abulk substrate, the first region of p-type semiconductor material, thefirst region of n-type semiconductor material, and the second region ofn-type semiconductor material are wells of a triple well structureformed in the bulk substrate, and, and further comprising: a pluralityof contact regions intersecting a top surface of the bulk substrate,each of the contact regions electrically connected with one of the wellsof the triple well structure.
 4. The device structure of claim 3 furthercomprising: a plurality of dielectric regions intersecting the topsurface of the bulk substrate, each of the dielectric regions positionedlaterally between an adjacent pair of the contact regions.
 5. The devicestructure of claim 1 wherein the second region of p-type semiconductormaterial is a p-type bulk substrate, the second region of n-typesemiconductor material is a first n-well in the p-type bulk substrate,the first region of p-type semiconductor material is a p-well in thep-type bulk substrate that surrounds the first n-well, and the secondregion of n-type semiconductor material is a second n-well in the p-typebulk substrate that surrounds the p-well.
 6. The device structure ofclaim 5 wherein the p-type bulk substrate has a top surface, the firstn-well extends laterally beneath the p-well, and further comprising: ann-type conductive reachthrough region extending from the top surface ofthe p-type bulk substrate to contact the first n-well.
 7. The devicestructure of claim 1 wherein the first region of n-type semiconductormaterial is a cathode of the vertical NPN bipolar transistor, the secondregion of n-type semiconductor material is an anode of the vertical NPNbipolar transistor, the first region of p-type semiconductor material isa base of the vertical NPN bipolar transistor, and the base of thevertical NPN bipolar transistor is shorted to the cathode of the of thevertical NPN bipolar transistor.
 8. The device structure of claim 7wherein the second region of p-type semiconductor material is an anodeof the vertical PNP bipolar transistor, the second region of n-typesemiconductor material is a base of the vertical PNP bipolar transistorthat is shared with the anode of the vertical NPN bipolar transistor,the second region of p-type semiconductor material is a cathode of thevertical PNP bipolar transistor that is shared with the base of thevertical NPN bipolar transistor, and the base of the vertical PNPbipolar transistor is shorted to the anode of the vertical PNP bipolartransistor.
 9. The device structure of claim 1 wherein the vertical NPNbipolar transistor and the vertical PNP bipolar transistor areconfigured as a breakdown-triggered silicon-controlled rectifier (SCR).10. A method of fabricating a device structure, the method comprising:forming, in first active region of a p-type bulk substrate, an NPNPstructure containing a first n-type semiconductor region, a first p-typesemiconductor region, and a second n-type semiconductor region that arearranged in depth to define a first vertical NPN bipolar transistor anda first vertical PNP bipolar transistor, the p-type semiconductor regionshared between the first vertical NPN bipolar transistor and the firstvertical PNP bipolar transistor, and the second n-type semiconductorregion shared between the first vertical NPN bipolar transistor and thefirst vertical PNP bipolar transistor; forming a first wiring path in aninterconnect structure and connected to the first n-type semiconductorregion and the p-type semiconductor region of the NPNP structure;forming an I/O pad connected by the first wiring path to the firstn-type semiconductor region and the p-type semiconductor region of theNPNP structure; and forming a second wiring path in the interconnectstructure configured to connect the second n-type semiconductor regionand the p-type semiconductor region of the NPNP structure to a positivepower supply voltage.
 11. The method of claim 10 wherein the firstvertical NPN bipolar transistor and the first vertical PNP bipolartransistor configured to direct an electrostatic discharge (ESD) pulseof negative voltage from the I/O pad to a power supply supplying thepositive power supply voltage.
 12. The method of claim 10 wherein thefirst vertical NPN bipolar transistor and the first vertical PNP bipolartransistor are formed in the p-type bulk substrate with CMOS triple welltechnology.
 13. The method of claim 10 further comprising: forming, in asecond active region of the p-type bulk substrate, a PNPN structurecontaining a first p-type semiconductor region, a first n-typesemiconductor region, a second p-type semiconductor region, and a secondn-type semiconductor region arranged in depth to define a secondvertical PNP bipolar transistor and a second vertical NPN bipolartransistor, the first n-type semiconductor region shared between thesecond vertical PNP bipolar transistor and the second vertical NPNbipolar transistor, and the second p-type semiconductor region sharedbetween the first vertical NPN bipolar transistor and the first verticalPNP bipolar transistor; forming a third wiring path in an interconnectstructure and connected to the first p-type semiconductor region and thefirst n-type semiconductor region of the PNPN structure; forming an I/Opad connected by the first wiring path to the first p-type semiconductorregion and the first n-type semiconductor region of the PNPN structure;and forming a second wiring path in the interconnect structureconfigured to connect the second p-type semiconductor region and thesecond n-type semiconductor region of the PNPN structure to a negativepower supply voltage.
 14. The method of claim 13 wherein the firstn-type region of the NPNP and the first n-type region of the PNPN areconcurrently formed by a first ion implantation process implanting ionsof an n-type dopant into the p-type bulk substrate.
 15. The method ofclaim 14 wherein the second n-type region of the NPNP and the secondn-type region of the PNPN are concurrently formed by a second ionimplantation process implanting ions of an n-type dopant into the p-typebulk substrate.
 16. The method of claim 15 wherein the p-type region ofthe NPNP and the second p-type region of the PNPN are concurrentlyformed by an ion implantation process implanting ions of a p-type dopantinto the p-type bulk substrate.
 17. The method of claim 10 wherein thefirst n-type semiconductor region is a first n-well in the bulksubstrate, the first p-type semiconductor region is a p-well in the bulksubstrate that surrounds the first n-well, and the second n-typesemiconductor region is a second n-well in the bulk substrate thatsurrounds the p-well.
 18. The method of claim 17 wherein the p-type bulksubstrate has a top surface, the first n-well extends laterally beneaththe p-well, and further comprising: forming an n-type conductivereachthrough region extending from the top surface of the p-type bulksubstrate to contact the first n-well.
 19. The method of claim 10wherein the first n-type semiconductor region is a cathode of thevertical NPN bipolar transistor, the second n-type semiconductor regionis an anode of the vertical NPN bipolar transistor, the first p-typesemiconductor region is a base of the vertical NPN bipolar transistor,and the base of the vertical NPN bipolar transistor is shorted to thecathode of the of the vertical NPN bipolar transistor.
 20. The method ofclaim 19 wherein the p-type substrate is an anode of the vertical PNPbipolar transistor, the second n-type semiconductor region is a base ofthe vertical PNP bipolar transistor that is shared with the anode of thevertical NPN bipolar transistor, the first p-type semiconductor regionis a cathode of the vertical PNP bipolar transistor that is shared withthe base of the vertical NPN bipolar transistor, and the base of thevertical PNP bipolar transistor is shorted to the anode of the verticalPNP bipolar transistor.
 21. A design structure embodied in a machinereadable medium for designing, manufacturing, or testing an integratedcircuit, the design structure comprising: an electrostatic discharge(ESD) protection device including a first region of n-type semiconductormaterial, a second region of n-type semiconductor material, a firstregion of p-type semiconductor material, a second region of p-typesemiconductor material, and an input/output (I/O) pad electricallyconnected to the first region of n-type semiconductor material and thefirst region of p-type semiconductor material, the first region ofp-type semiconductor material stacked vertically between the first andsecond regions of n-type semiconductor material to form a vertical NPNbipolar transistor, the first and second regions of p-type semiconductormaterial stacked vertically relative to the second region of n-typesemiconductor material to form a vertical PNP bipolar transistor, thefirst region of p-type semiconductor material shared between the firstvertical NPN bipolar transistor and the first vertical PNP bipolartransistor, the second region of n-type semiconductor material sharedbetween the first vertical NPN bipolar transistor and the first verticalPNP bipolar transistor, and the second region of n-type semiconductormaterial and the second region of p-type semiconductor materialconfigured to be electrically connected to a positive power supplyvoltage such that an electrostatic discharge (ESD) pulse of negativevoltage at the I/O pad is directed through the first vertical NPNbipolar transistor and the first vertical PNP bipolar transistor to apower supply supplying the positive power supply voltage.
 22. The designstructure of claim 21 wherein the design structure comprises a netlist.23. The design structure of claim 21 wherein the design structureresides on storage medium as a data format used for the exchange oflayout data of integrated circuits.
 24. The design structure of claim 21wherein the design structure resides in a programmable gate array.